Wideband code division multiple access (WCDMA) is a third generation (3G) cellular technology that enables the concurrent transmission of a plurality of distinct digital signals via a common RF channel. WCDMA supports a range of communications services that include voice, high speed data and video communications. One such high speed data communications service, which is based on WCDMA technology, is the high speed downlink packet access (HSDPA) service.
WCDMA is a spread spectrum technology in which each digital signal is coded or “spread” across the RF channel bandwidth using a spreading code. Each of the bits in the coded digital signal is referred to as a “chip”. A given base transceiver station (BTS), which concurrently transmits a plurality of distinct digital signals, may encode each of a plurality of distinct digital signals by utilizing a different spreading code for each distinct digital signal. At a typical BTS, each of these spreading codes is referred to as a Walsh code. The Walsh coded digital signal may in turn be scrambled by utilizing a pseudo noise (PN) bit sequence to generate chips. An example of a PN bit sequence is a Gold code. Each of a plurality of BTS within an RF coverage area may utilize a distinct PN bit sequence. Consequently, Walsh codes may be utilized to distinguish distinct digital signals concurrently transmitted from a given BTS via a common RF channel while PN bit sequences may be utilized to distinguish digital signals transmitted by distinct BTSs. The utilization of Walsh codes and PN sequences may increase RF frequency spectrum utilization by allowing a larger number of wireless communications to occur concurrently within a given RF frequency spectrum. Accordingly, a greater number of users may utilize mobile communication devices, such as mobile telephones, Smart phones and/or wireless computing devices, to communicate concurrently via wireless communication net works.
A user utilizing a mobile communication device, MU1, may be engaged in a communication session with a user utilizing a mobile communication device MU2 via a base transceiver station, BTSA within wireless communication network. For example, the mobile communication device MU2 may transmit a digital signal to the BTSA, which the base transceiver station BTSA may then transmit to the mobile communication device MU1. The base transceiver station BTSA may encode signals received from the mobile communication device MU2 and transmitted to the mobile communication device MU1 by utilizing a Walsh code, W12, and a PN sequence, PNA. The mobile communication device MU1 may receive signals transmitted concurrently by a plurality of base transceiver stations (BTSs) in addition to the base transceiver station BTSA within a given RF coverage area. The mobile communication device MU1 may process the received signals by utilizing a descrambling code that is based on the PN sequence PNA and a despreading code that is based on the Walsh code W12. In doing so, the mobile communication device MU1 may detect a highest relative signal energy level for signals received from base transceiver station BTSA, which comprise a digital signal corresponding to mobile communication device MU2.
However, the mobile communication device MU1 may also detect signal energy from the digital signals, which correspond to signals from mobile communication devices other than the mobile communication device MU2. The other signal energy levels from each of these other mobile communication devices may be approximated by Gaussian white noise, but the aggregate noise signal energy level among the other mobile communication device may increase in proportion to the number of other mobile communication devices whose signals are received at the mobile communication device MU1. This aggregate noise signal energy level may be referred to as multiple access interference (MAI). The MAI may result from signals transmitted by the base transceiver station BTSA, which originate from signal received at the base transceiver station BTSA from mobile communication devices other than mobile communication device MU2. The MAI may also result from signals transmitted by the base transceiver stations BTSs other than the base transceiver station BTSA. The MAI and other sources of noise signal energy may interfere with the ability of MU1 to successfully decode signals received from MU2.
An additional source of noise signal energy may result from multipath interference. The digital signal energy corresponding to the mobile communication device MU2, which is transmitted by the base transceiver station BTSA, may disperse in a wavefront referred to as a multipath. Each of the components of the multipath may be referred to as a multipath signal. Each of the multipath signals may experience a different signal propagation path from the base transceiver station BTSA to the mobile communication device MU2. Accordingly, different multipath signals may arrive at different time instants at the mobile communication device MU2. The time duration, which begins at the time instant that the first multipath signal arrives at the mobile communication device MU2 and ends at the time instant that the last multipath signal arrives at MU2 is referred to as a delay spread. The mobile communication device MU2 may utilize a rake receiver that allows the mobile communication device MU2 to receive signal energy from a plurality of multipath signals received within a receive window time duration. For a given rake finger, which receives signal energy from a selected multipath signal, the given rake finger may also receive noise signal energy from other multipath signals in the multipath.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.